Helping Pea Patch Island stand up to waves

Rachel Schaefer explores how vegetation can reduce erosion of Delaware River island

Rachel Schaefer is a senior, majoring in civil engineering, from West Windsor, New Jersey.

Q. What are you studying, where and with whom?

Schaefer: I’m studying the effects of marsh vegetation located on the south side of Pea Patch Island on incoming waves. This is for my senior thesis under the advisement of Prof. Jack Puleo. I’m also working with Michael Larner, who will write a senior thesis on how ship wakes affect the bare beach on the north side of Pea Patch Island. We’re also working with Prof. Thomas McKenna.

Pea Patch Island, which contains Fort Delaware and a nature preserve, is on the Delaware River and can be reached by a boat ride from Delaware City. I set out sensors in early June to measure water depths and velocities along a transect before and within a patch of vegetation. As the vegetation grows, the degree of wave attenuation could potentially increase. I have been conducting surveys of the transect on a regular basis to measure how the ground elevations change over time. Most researchers who have published on this topic performed controlled experiments in wave flumes, which allowed them to manipulate some variables but could not capture all of the environmental factors present in a real marsh. I plan to also explore numerical modeling of vegetation attenuation of water waves, and perhaps compare a model simulating the Pea Patch Island marsh to my data analyses.

Q. Why is this work important?

Schaefer: Many large ships bearing heavy cargo pass Pea Patch Island and generate powerful wakes that slam into the island. Vegetation is known to reduce erosion by attenuating waves and stabilizing the surrounding sediment.

Q. What is it about this topic that interests you?

Schaefer: I’m fascinated by how the interactions between plants, sediment and water play out on a small scale but have enormous implications and potential uses. While in Malawi, Africa, in 2016 implementing a water project with the University of Delaware chapter of Engineers Without Borders, I saw evidence of extensive deforestation and the devastating effects of a flood followed closely by a drought. The absence of trees and plants allows water to more quickly flow and accumulate, and allows sediment to move more freely and fill rivers. Most electricity in Malawi is generated through hydropower, and a large portion of the population depends on fish from the huge Lake Malawi. Removing vegetation near sources of water can have multiple cascading consequences.

Last year, I did a class project that involved designing subsurface flow constructed wetlands to treat wastewater for a community in the Philippines. From working on that project, I became even more interested in how water and particles can interact with plants.

I’m also interested in how plants at the interface between wetlands and water can combat sea level rise due to climate change. Wetland vegetation may only partially decay, forming layer upon layer of peat over time. The buildup of peat may compete with a rising sea level, while the peat also stores carbon dioxide that would otherwise be released into the atmosphere. Vegetation can also trap sediment washed in, further building up the land to reduce the effects of sea level rise while improving water quality. Plants may seem like part of the scenery, but they play crucial roles in marshland and coastal environments.

Q. What is a typical day like?

Schaefer: Many days have involved getting to work at 6 a.m. to gather equipment and catch a crew boat to the island. While there, we’re downloading data from some sensors, swapping batteries, as well as taking measurements, vegetation samples, notes and photographs. Bald eagles, herons, egrets, geese, muskrats and countless biting insects provide a constant audience.

I spend other days at the Ocean Engineering Laboratory where I read literature, process and analyze data, and watch footage downloaded from a time lapse camera I set up near my sensors to note ships coming by and subsequent ship wakes. I work with Mike to set aside equipment, charge batteries and prepare for upcoming days on Pea Patch Island. I may ask a graduate student or Prof. Puleo questions about how I’m using MATLAB to process the data, or about equations and concepts I found in my reading. In the background, I usually hear some students performing experiments in the wave flume. I’ve also been helping Prof. Puleo on an outreach project involving the construction and delivery of mini wave flumes to high schools along the East Coast. We are developing a teaching module where students can run waves and collect data to learn how vegetation can attenuate waves.

Q. What is the coolest thing you’ve gotten to do on the project?

Schaefer: Deploying water depth and velocity sensors for data collection at the beginning of June. Designing the experiment, preparing equipment, programming sensors and deploying them to take measurements felt like leaping into the unknown. Watching waves wash over and under my sensors, hearing the acoustic distance meters send out pulses, is uniquely satisfying. It’s easy to say “just go get some data,” but difficult to actually prepare for and execute a data collection. Leading up to the deployment, I felt increasingly buried by logistical details, but being able to finally get my hands dirty and my feet wet was amazing. Hearing the mud squelch as I drove scaffolding pipes into the ground to support the sensors made me smile. As I rode away from the island on a boat after the deployment, looking at my line of data collection stations made the project feel so real to me.

Q. What has surprised you the most about your experience?

Schaefer: I was surprised by how, after I defined the main research questions I was investigating, as I dug deeper, more and more questions came up. It reminded me of the Phragmites australis in the patch I’m studying, where a single seed can grow up into a 15-feet-tall plant, developing complex rhizome systems that grow more plants. Each mature plant bears long, broad leaves and a fluffy seedhead. I started out with the intention to study vegetation wave attenuation, the seed to the project. The rhizome system grew as I expanded my base of knowledge of the topic, and sprouted new plants when I focused on specific aspects of it. Ship wakes, sediment deposition, peat cliffs, season, vegetation species, as well as the height, width, shape, density, strength and leafiness of the plant stems were just a few of the offshoot questions that grew. Entire papers have been published on each individual subtopic I come across. I must investigate the involvement of each variable while keeping in mind the big picture.

Q. Dreaming big, where do you hope this work could lead?

Schaefer: I hope this work leads to DNREC implementing more natural coastal protection solutions and planting marsh grass along the edges of beaches and coastal areas. Delaware is vulnerable to sea level rise, so strategic planting of vegetation could help mitigate its effects. As shown on Pea Patch Island, rock seawalls are effective at protecting the immediate area behind them but may cause erosion further down the shoreline as the natural transport and replenishment of sediment there is restricted. As mentioned above, plants make many contributions to coastal ecosystems, certainly more than rocks.

Pea Patch Island attracts many visitors, and as they walk down the dock from the ferry they may look to their right and see my line of sensors going into the vegetation. I hope the sight of my sensors has caused visitors to wonder and think about what I was measuring and why I was measuring, perhaps inspiring them to better appreciate and respect what plants do.

Q. If you had to summarize your experience in only one word, what would it be?

Schaefer: Humbling. Solving problem sets in class with predetermined parameters and solutions gives me and other students a false sense of confidence and knowledge, when in reality no actual problem comes with neatly defined variables and a clear path to an answer. As I worked on Pea Patch Island, I watched many huge ship wakes roll in, wave after wave slamming into the shore, and felt rather small in comparison. Small animals like geese to massive freighters generate similar wake patterns. There is a rhythmic and timeless feel to the motions of the waves and tides. Cliffs of wetland peat represent decades to centuries of plant birth and decay. And here I am trying to take measurements and make sense of these complex and multifaceted interactions. Although the physics and math often seem daunting and difficult to manipulate, I am probably only scratching the surface.

Q. What do you enjoy when you are not doing research?

Schaefer: When I’m not doing research I like to wander around and explore the local area, often going out without any predetermined plans and taking whatever paths strike my interest. I enjoy reading classic novels and watching science fiction shows. Sometimes I write poetry or chip away at stories I’ve been developing for a while. I love spending time with friends and visiting family members and pets.